Radio Frequency Identifier (RFID) tags are used in a variety of applications, such as inventory control and security. Unlike barcode tracking systems, the advantage of these more intelligent RFID systems is that an RFID system can store specific information about an article and can read that information on a tag without requiring line of sight or a particular orientation. This means that RFID systems can be largely automated, reducing the need for manual scanning.
These RFID tags are typically placed on or in articles, or containers such as cardboard boxes. The RFID tags work in conjunction with an RFID base station. The base station supplies an electromagnetic wave output, which acts as the carrier frequency. Data are then used to modulate the carrier frequency to transmit specific information. RFID systems typically operate at either a low frequency range (generally less than 100 MHz), or a higher frequency range (greater than 100 MHz). In many applications, one such higher frequency range is between 800 and 1000 MHz (defined as the UHF Band), with 915 MHz being the most common high frequency currently utilized in the United States. Most RFID systems utilize frequency hopping centered around this frequency, such that the overall frequency range is approximately 902 to 928 MHz. A second high frequency used by RFID tags in the United States is 2450 MHz. Currently, European standards utilize 869 MHz and the Japanese standard is 953 MHz.
Many RFID tags contain integrated circuits, which are capable of storing information. Depending on the specific implementation of the RFID tag, the integrated circuit may be capable of replacing stored information with new information at a later time. When the base station requests data, the integrated circuit supplies the information that it has stored in response to that request. In those RFID tags that permit information to be rewritten, the integrated circuit overwrites its existing information when new data are received from the base station.
In addition to the integrated circuit, the RFID tags contain an antenna. The antenna is needed to receive the electromagnetic waves generated by the base station, and to transmit data via the same frequency. The configuration of the antenna can vary, and includes flat coils, patches, microstrip antennas, stripline antennas and dipoles.
Some of these RFID tags are self-powered, that is, they contain an internal power supply such as a battery. Other RFID tags are field-powered. These latter tags use incident RF energy transmitted by the base station to supply their required voltage. The RF energy is received by the tag antenna as an AC signal, which is then rectified to form a DC voltage, which is used to power the integrated circuit.
These integrated circuits have a minimum voltage requirement below which they cannot function and the tag cannot be read. The rectified DC voltage is a function of the signal strength of the received electromagnetic wave. For example, a RFID tag that is proximate to the base station will receive more energy and therefore be able to supply sufficient voltage to its integrated circuit, as contrasted to a RFID tag which is physically farther away from the base station. The maximum distance between the base station and the RFID tag at which the RFID tag can still be read is known as the read distance. Obviously, greater read distances are beneficial to nearly all RFID applications.
One benefit of RFID tags that operate in the high frequency range is the potential to have much greater read distances than tags operating at low frequency. RFID tags utilizing the 915 MHz frequency range typically possess a read distance in excess of 10 feet in free air. In contrast, lower frequency (such as 13.56 MHz, which is part of the HF Band) tags rarely achieve read distances greater than 2 feet.
One reason for this difference is due to the difference in the energy transfer mechanisms at the HF and UHF frequencies. As described above, it is the electric field of the propagating signal that gives rise to a potential difference across the antenna at UHF frequencies. In contrast, passive RFID tag systems operating in the HF frequency band at 13.56 MHz employ magnetic induction to couple the transponder tag and the reader. The power required to energize and activate the HF tag microchip is drawn from the oscillatory magnetic field created by the reader.
Unfortunately, high frequency RFID tags cannot be read when the tag is in close proximity to a metal substrate or a substrate with high water content. Thus, an RFID tag attached to a metal container or to a bottle containing a soft drink cannot be read, from any distance.
Experimentation in the industry has shown that such RFID tags are once again readable if there is a substantial air gap interposed between the tag and the article substrate. This required air gap is typically at least one quarter of an inch or greater. Various designs have been developed to allow tags to “stand off” from the article substrate in order to create this gap. However, standoff tags are impractical in the majority of commercial applications. The distance between the tag and the article increases the likelihood of the tag being dislodged or damaged in normal use.
Recognizing that an air gap acts as a dielectric insulator, tag manufacturers have attempted to solve the stand off problem by interposing a thin layer of a dielectric insulating material of dielectric constant, k, between the tag and the article substrate. U.S. Pat. No. 6,329,915 discloses the use of a homogeneous material of high dielectric constant to address this issue. However, homogeneous materials with various k values have been tried with little or no success.
Therefore, a system and method for allowing the use of RFID tags on these substrates would represent a significant advance for the use of high frequency RFID tags.